A Few Quick Vacuum Questions

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Duncan Wilkie
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A Few Quick Vacuum Questions

Post by Duncan Wilkie » Thu Dec 28, 2017 1:38 am

Ok so I’m pretty close to having all the components of a working fusor on hand and had a few questions.

1: Can I use only a ball valve in the forline? It seems that flow rate doesn’t need to be controlled here. Would a valve for plumbing suffice or are there vacuum-rated ball valves? Also, how can I find foreline hoses that will withstand the 14ish psi of external pressure? Ones rated for that I would imagine are only designed for internal pressure.

2: I don’t know if this is silly, but how tight does a cf flange need to be? Do I need to replace the viton gaskets on my used diff pump, or can I recycle?

3: Assuming my system has some sort of serious leak (these things always do), how would I go about isolating and repairing it?

On a different note, I acquired a pneumatic gate valve in an eBay auction for $.99. It’s probably worthless to me, as I don’t have the equipment to control it, but if anyone knows how to use it, I would welcome that knowledge.

Thanks for your input.
Some say the glass is half full. Others see it as half empty. I say it is twice as big as it needs to be.

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Richard Hull
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Re: A Few Quick Vacuum Questions

Post by Richard Hull » Thu Dec 28, 2017 4:25 am

Ball valve only in foreline between mechanical and diff pump is OK

Viton gasket on diff pump can often be reused, but suggest a new one.

there is a FAQ on leaks.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: A Few Quick Vacuum Questions

Post by ian_krase » Thu Dec 28, 2017 7:45 am

Some more info:

- If you have a turbo-drag pump (i.e. most turbo pumps in the last 10 years), then ball valves are almost certainly OK since turbo drag pumps can tolerate quite a poor foreline vacuum. With diff pumps, things are harder. You may be able to get away with them, or you may not -- my first vacuum system had a (horrible low quality) ball valve that leaked enough to keep me at 100+ microns, too much for a diffpump.

- CF flanges with rubber gaskets are tightened till all the bolts are tight (use crisscross tightening order) and the flanges are touching. With copper gaskets as is proper, it ends up being a bit more qualitative. In both cases this works out to "pretty tight")

- There are three main types of foreline hoses. ("Vacuum tubing"). The first and cheapest is made of gum rubber, with a wall nearly as thick as the hole in the middle of the hose. The second is made of clear plastic and has a metal spring going through it to to keep it from collapsing. The third is just a stainless steel bellows hose -- these are also usable in high vacuum, but very expensive and either stiffer than you expect or more fragile than you want.

If you have a leak that is too small to see/hear, then you need a can of Dust-Off and some kind of gauge appropriate for the vacuum you can achieve, either an ion gauge, a thermocouple/pirani gauge, or just a visible plasma discharge. Spray Dust-Off around places where you think the leak might be, and wait for the gauge reading to change or the plasma to change color and appearance (turns whiter and looks like pressure is changing).

(People keep saying that Dust-Off is not very practical with pirani gauges. They are wrong. I do this routinely.)

You may also "tent" small areas of your aparatus with plastic wrap before spraying Dust-Off. However this usually isn't needed.

Likely flaws: Bad gaskets (esp. if dust or a hair (cat or human) is across the o-ring), cracks in welds, things you cheated on with grease turning out to be leaky again, valves that are not bellows sealed, cracks in bellows, cracks in feedthroughs, etc.

Post a picture of your gate valve, and tell us any numbers or data printed / stamped on it. It is quite possible that it might be useful. Stuff valve cylinders can be run by an extremely small compressor.

Jerry Biehler
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Re: A Few Quick Vacuum Questions

Post by Jerry Biehler » Sat Dec 30, 2017 6:18 am

There are also high vacuum rated ball valves, but you are not going to find these much anymore. But a foreline you should be ok.

If your DP has viton seals on the CF flange you can reuse them. They are too expensive not too. Otherwise copper or aluminum crush gaskets for CF are one time use. You squish them down till the flange meet. Technically you could get two uses out of them if you were very careful and squished down half way but for smaller gaskets it is just not worth messing around.

Personally for people with CF flange stuff on a fusor I would just get the viton seals for the conflat. You will never see the benefit of CF at such a low vacuum.

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